I cannot install libdb4.8-dev + libdb4.8, because it conflicts with libdb5.1.

This does not seem to be true, the dev packages conflict but afaict the
libraries themselves (at least the versions from debian squeeze and
wheezy) do not. So as long as you don't need libdb5.1-dev installed you
should be fine.

If you need to use other dev packages that depend on the db dev packages
to build bitcoind then I would suggest building it in a squeeze chroot.
Once you have built it it should be easy enough to install required
binary libraries on your wheezy system as unlike dev packages they
rarely conflict.